A hypothesis is proposed which envisages the polyamines, spermidine and spermine and the diamines, putrescine and histamine, as representative of a group of compounds acting as regulators of cell metabolism. According to this view each amine has its own specific function and the differential distribution of these compounds throughout the animal kingdom is thought to reflect different growth patterns with differing regulatory requirements. Particularly critical is the implication that putrescine, which is often regarded simply as a precursor for spermidine, is itself biologically functional. This will be investigated initially on dynamic grounds by metabolic studies comparing the rate and extent of the incorporation of putrescine into spermidine with alternate routes of metabolism such as oxidative deamination via diamine oxidase, tissue binding and release. The rat placenta, which contains very high levels of the putrescine forming enzyme, ornithine decarboxylase, is particularly suitable for the study. Subsequently, the dynamics of diamine metabolism will be studied in the placenta of other species and of direct relevance to the hypothesis is the isolation and identification of the diamine expected to be present in the human placenta. These studies will define more closely the relationship between diamine metabolism and tissue growth and by pursuing a comparative approach overcome the practical difficulties of establishing the role of these substances in man. Since the diamines, and by inference the polyamines, are largely concentrated in the reproductive tract of both sexes, in embryonic tissues and in malignant growth, then the findings are relevant to the fields of both reproductive physiology and cancer.